As one of the methods of charging a secondary battery, there is constant voltage charging. In this method, a constant voltage is applied from a charging unit to the secondary battery. In the charging starting period, large current flows, and in the charging ending period, the charging current becomes small. Recently, a high-voltage electricity storage device such as an electricity storage device of a house or the like and a battery for an electric car has been used, and a battery system having, as a unit, a battery pack which is configured by serial connection of secondary batteries (appropriately referred to as battery cells) or sub modules (sometimes, referred to as battery blocks) configured by parallel connection of a plurality of battery cells has been used.
For example, a battery pack configured by serial connection of a predetermine number (for example, 16) of battery cells may be charged by constant voltage charging. In this case, the charging is performed until the voltage of the entire battery pack is substantially equal to a charging voltage. In the battery cells included in the battery pack, there is a variation in characteristics. Therefore, the voltages of the battery cells are not equally increased.
In the case of charging the battery pack configured by serial connection of battery cells, it is necessary to prevent the battery cells from being over-charged. For example, PTL 1 discloses a charging device which prevents over-charging. As disclosed in PTL 1, the charging device monitors each voltage of serially-connected battery cells, and if a voltage of any one of the battery cells reaches a predetermined voltage, for example, 4.2 V (maximum charging voltage of a lithium ion secondary battery), the charging device decreases the charging current stepwise.